1. Technical Field
The present invention relates to a correlation calculating method, a satellite signal capturing method, and a correlation calculating circuit.
2. Related Art
GPS (Global Positioning System) has become widely known as a positioning system using a positioning satellite signal and has been used for a position calculating device built in a mobile phone or a car navigation apparatus. The GPS performs a position calculating operation of calculating positional coordinates and a clock error of a position calculating device on the basis of information such as positions of plural GPS satellites or pseudo-distances from the GPS satellites to the position calculating device.
A GPS satellite signal transmitted from a GPS satellite is modulated with a spread code which is called a CA (Coarse Acquisition) code varying depending on the GPS satellites. To capture the GPS satellite signal from weak signals received, the position calculating device performs a correlation calculation on the received signal of the CA code (received code signal) and a signal of a replica code (replica code signal) which is obtained by replicating the CA code to capture the GPS satellite signal.
As a specific circuit configuration used to capture the GPS satellite signal, for example, a configuration in which plural correlators are arranged in parallel to form a correlation calculating circuit and the correlators are made to perform a correlation calculation in parallel while varying the phase of the replica code signal is used (for example, see JP-A-2001-159670).
In the related art, the correlation calculating circuit is made to perform a correlation calculation between sampled values of the received code signal sampled on the basis of the a sampling signal and sampled values of the replica code signal, for example, continuously during one period of the CA code and it is determined on the basis of the summed value thereof whether the capturing of a signal is successful. The code length of the CA code is 1023 chips. Accordingly, for example, when the sampling is carried out with a period of ½ chip of the CA code, 1023×2=2046 multiplications are necessary for the correlation calculation during one period of the CA code and it is determined on the basis of the summed value of 2046 calculated values whether the capturing of a signal is successful.
Since the CA code is a so-called gold code, the summed value is high only when the capturing of a signal is successful. Accordingly, it can be determined that the signal could be captured, that is, the phase (code phase) could be specified, when a high value is obtained by performing the same process on different phases while varying the phase of the replica code signal. The variation in phase of the replica code signal is embodied, for example, by causing sampling positions as a correlation calculation target to depart from each other one by one. For example, when the sampling is carried out with a period of ½ chip, the number of sampling positions is 2046. Therefore, when it is intended to perform the correlation calculation with one period of the CA code on all the phases, 2046×2046 multiplications are necessary.
Therefore, there is a problem in that the correlation calculation amount increases exponentially as the sampling interval is shortened. In order to shorten the sampling interval, it is also necessary to generate higher-frequency sampling signals. This causes an increase in power consumption of a circuit receiving a positioning satellite signal.